Life is stressful. Cells are repeatedly exposed to various stressors that disrupt protein homeostasis (proteostasis), resulting in protein misfolding and aggregation. To maintain proteostasis, cells have evolved compartment-specific stress responses aimed at repressing translation, inducing chaperone expression, and eliminating damaged proteins. The endoplasmic reticulum (ER) is responsible for the folding of nearly one third of the total human proteome, including almost all of the proteins that are secreted. To protect the function of this essential organelle, the ER can initiate the unfolded protein response (UPRER) in response to the presence of misfolded proteins. Our laboratory examined the role of the UPRER in aging within the nematode C. elegans and found that the ability to activate the IRE-1/XBP-1 pathway in response to ER stress is abrogated with age. This reduction in UPRER diminishes protection against ER stress-inducing agents. Interestingly, this age-dependent decline in UPRER-mediated protection against ER stress can be prevented by the expression of constitutively active XBP- 1s in the nervous system. Most striking, neuronal expression of XBP-1s in C. elegans leads to cell non- autonomous activation of the UPRER in distal cell types and increases lifespan. It was recently reported that overexpression of XBP-1s in neurons in the hypothalamus induces XBP-1 splicing in the liver and protected mice from diet-induced obesity. Therefore, cell non-autonomous stress signaling is conserved between C. elegans and rodents and regulates key metabolic functions. We hypothesize that neuronal XBP-1s induces a trans-cellular signaling mechanism to coordinate an organism-wide stress response, an improved metabolic state, and a longer lifespan. However, the genetic components required for this signaling mechanism are unknown. In the novel CRISPR-Cas9 genetic screen proposed in aim 1, we will identify essential mediators of the XBP1s transcellular stress response that will provide additional, potentially more specific, therapeutic targets for aging and metabolic disease. Furthermore, we hypothesize that neuronal XBP1s overexpression in mammals will rescue the age-onset loss of UPRER and increase longevity. In Aim 2 we will test our hypothesis using transgenic mice that overexpress XBP1s in hypothalamic neurons. Since one of the greatest challenges facing our healthcare system is the growing economic burden of age-related metabolic and neurologic diseases, elucidating the mechanisms that regulate XBP1s-induced longevity and determining the role of XBP1s in mammals are of paramount significance. Answering the questions described in this proposal will have enormous therapeutic implications not only for normal aging, but also age-onset diseases.